Interpolymers of ethylene and one or more polar monomers, such as acrylic acid, methacrylic acid and vinyl acetate, are well known. Such interpolymers are commonly produced, sold and shipped in pelletized form, or as plural extrudates. However, the high block, or the tendency to stick or clump together, of pellets made of such interpolymers has been a serious problem in transporting these products, both locally and to remote destinations, particularly with such interpolymers containing a relatively high proportion of interpolymerized polar monomer.
One attempt at solving this problem, when the polar monomer was an .alpha.,.beta.-ethylenically unsaturated carboxylic acid such as acrylic acid, was to surface neutralize the carboxyl groups of the interpolymer pellets or chips with a basic solution such as aqueous alkali metal hydroxide or carbonate, ammonia, or aqueous amine, as described in U.S. Pat. No. 3,753,965. However, over a period of time, typically about 3 months, the neutralizing cation migrated from the surface to the interior of the pellet or chip, reactivating the surface acid groups. In addition, any surface distortion, such as might occur from erosion or breakage of the pellets during pneumatic conveying, or upon repeated heating and cooling, exposed unneutralized acid groups, resulting in an increased tendency of the pellets to clump together. Moreover, if an excessive fraction of the acid groups were neutralized, properties of the interpolymer, such as melt flow value, strength, stiffness, hardness, and softening point, were substantially adversely affected.
Another attempt was the employment of powdered anti-clumping agents such as talc, calcium carbonate, and the like. When such materials were either intimately blended with the interpolymer or admixed with the pellets in an effective amount, the properties of the interpolymer, such as stiffness and, in the case of aqueous dispersible grade resins, dispersibility, were substantially adversely affected. Moreover, it was difficult to accurately admix these additives in the desired proportions and to obtain sufficient mixing to ensure that the surfaces of the pellets were adequately coated.
Other references which may be relevant to this application include: U.S. Pat. Nos. 3,595,827; 3,935,124; 2,882,254; 3,324,060; 3,901,992; 3,916,058; 4,129,717; and 3,937,676.